Application of nitride semiconductors to semiconductor devices with high breakdown voltage and high output power is being studied by utilizing characteristics such as high electron saturation velocity and wide band gap. For example, the bandgap of gallium nitride (GaN) is 3.4 eV, which is greater than the band gap (1.1 eV) of silicon (Si) and the band gap (1.4 eV) of gallium arsenide (GaAs), and has high breakdown electric field. Therefore, nitride semiconductors such as GaN are highly promising as materials for power supply semiconductor devices to be operated at high voltage and to obtain high output power.
As semiconductor devices using nitride semiconductors, there have been many reports on field effect transistors, particularly high electron mobility transistors (HEMTs). For example, in GaN-based HEMTs (GaN-HEMTs), attention has been paid to HEMTs using GaN as an electron transit layer and AlGaN as a barrier layer or HEMTs using InAlGaN as a barrier layer. Specifically, HEMTs using InAlGaN as the barrier layer have two-dimensional electron gas (2DEG) with a density two to three times higher than that of HEMT using AlGaN as the barrier layer, which is advantageous for increasing the output power of the device. Further, in general, a gate length of the device is reduced in order to implement a large current operation.
Meanwhile, in the HEMT using the nitride semiconductor, as a drain voltage is increased, a state where the gate is below the pinch-off voltage, that is, an off-leakage current bypassing below the gate electrode may flow despite the transistor being in an off-state.
In GaN-HEMT, in order to grow GaN or the like on a substrate such as sapphire or silicon carbide (SiC), a buffer layer is formed on the substrate, and a GaN crystal is formed on the formed buffer layer. The thus formed buffer layer exhibits poor crystal quality; such a buffer layer may contain a large amount of impurity elements or may include lattice defects such as point defects and dislocations at high density.
Since these impurities and crystal defects in the buffer layer form an electron trap level in the buffer layer, the electron traps capture and release electrons during transistor operation, leading to current collapse. Specifically, electrons are susceptible to becoming injected into the buffer layer where located below the gate electrode, and the buffer layer below the gate electrode frequently causes a large current collapse.
As a structure for reducing both such off-leakage current and current collapse, a method for removing a semiconductor layer where below the gate electrode from the back surface (bottom surface in the figure) to form a groove, a method for embedding this groove with SiN, etc., is disclosed.